The present invention relates to a process for regenerating spent bleaching earth, as well as to the use of said regenerated bleaching earth for bleaching. The term spent bleaching earth as used herein is intended to describe spent bleaching earth resulting from glyceride oil, or its derivatives, or paraffine refining processes. The term glyceride oil as used herein is intended to encompass both vegetable and animal oils, in particular the so-called edible oils, i.e. oils used chiefly in foodstuffs, but it has to be understood that oils of which the end use is non-edible are to be included as well.
In edible oil refining, the bleaching process is generally considered to be of critical importance in determining the quality and stability of the final product. Although bleaching was originally intended for the removal of coloring substances from the oil, it is now recognized that this process is responsible for the removal of a whole range of impurities.
Besides decolorization, the most important purpose of this bleaching process is the removal of trace elements such as iron, copper, calcium, magnesium, nickel and phosphorus, some of which elements are known to promote oxidation of the oil and to limit the storage stability of the oil if they are not removed. These trace elements may be associated with phospholipids, but also with fatty acids in the form of soaps, especially when the oil results from caustic refining or hydrogenation. Bleaching of edible oils or fatty acids is also carried out to remove constituents that deactivate hydrogenation catalysts and thus lead to a more reproducible hydrogenation process and product, and to reduced catalyst usage.
Bleaching of edible oils generally entails mixing the oil with an appropriate amount (e.g. 0.2-2.0%) of adsorbent at increased temperature (e.g. 70.degree.-140.degree. C.), stirring the mixture--preferably under reduced pressure and in the absence of oxygen--and filtering e.g. through a filterpress (Bailey's Industrial Oil and Fat Products, 4th Edition, Volume 2, 1982, John Wiley & Sons, New York, pages 27 to 52).
In order to minimize oil losses, the filter cake resulting from the removal of bleaching earth from the oil stream is commonly blown with steam. By doing so, the oil content of the spent bleaching earth is reduced to 30-40 wt %. Nevertheless, the spent bleaching earth thus obtained presents a fire hazard because the oil may ignite spontaneously, especially when the oil is highly unsaturated. If disposal of this spent bleaching earth by landfill is contemplated, it must therefore be covered with earth to exclude contact with air as much as possible.
Landfill however, also presents environmental problems so that other ways of disposal of spent bleaching earth have been investigated. They usually involve introducing the spent earth into an extraction unit for edible oils or mixing the spent earth with the meal resulting from oil seed extraction. They have the disadvantage of meal adulteration and cannot be used when the spent bleaching earth contains nickel. Besides, many refineries do not have easy access to an oil crushing plant.
Accordingly, processes have been developed that aim at removal of the oil from the spent bleaching earth thereby generating an oil-free residue that can safely be disposed of by landfill. One of these processes (U.S. Pat. No. 2,706,201) involves a treatment with aqueous alkali, another (IT 532,224) involves an extraction with organic solvents and a third one the introduction of the spent earth into a biogas reactor. This last process is, however, sensitive to traces of nickel and can therefore not be used for spent bleaching earth resulting from bleaching of hydrogenated oils.
These treatments have in common that after treatment, the earth still has to be disposed of by e.g. landfill. Consequently, it is not surprising that attempts have been made to treat spent bleaching earth in such a way that a useful product results from this treatment. The easiest way to get rid of spent bleaching earth is to burn it for instance in a cement kiln to recover its energy content.
Another attempt has been described by Kazimierz Grabas, Marian Rutkowski and Bogumil Perzynski (Carbon-Mineral sorbent from spent bleaching earths, Przem. Chem., 1988, 3, 121-124), and a similar process has been disclosed in British Patent specification GB 684,036. Spent bleaching earth is heated in a multi-shelf furnace with a controlled admission of air. As a result, the oil present in the spent bleaching earth pyrolyzes and forms cracking gases and a carbon residue that coats the clay particles. The resulting carbon-mineral adsorbent can advantageously be used for e.g. the decolorisation of phosphoric acid, but not for the bleaching of edible oils or fats.
Attempts have also been made to regenerate spent bleaching earths for repeated use in the refining of edible oils.
In U.S. Pat. No. 1,851,627 a process is described wherein finely divided spent earth, such as spent bleaching earth, is treated with a suitable solvent to remove the bulk of the oil and is then mixed with a suitable quantity of water so as to form a mass which under relatively high temperature generates steam at a rate sufficient and in a sufficient quantity to make the mass porous and at the same time to decrease its density. Thereafter the mass is fed into a furnace where it is subjected to a temperature from 260.degree. to 815.degree. C. and then to a multiple shelf furnace where it is subjected to temperatures in the range of 371.degree. to 871.degree. C. in the presence of air so regulated as to secure proper combustion of the carbonaceous materials contained in the earth.
In British Patent specification No. 491,338 a method is disclosed for the regeneration of spent bleaching earths, which method comprises solvent extraction to remove the bulk of the oil and fat, steaming to remove the extraction solvent and heating the material in the presence of air to a temperature of 500.degree. to 550.degree. C. whilst being thoroughly stirred or agitated to facilitate uniform heating and action of the air until combustion gases are no longer evolved.
Yet another process is described in British Patent specification No. 1 278 379. In this process, spent bleaching earth is first of all extracted with an organic solvent. In a second stage, the solvent is removed from the extracted earth with steam whereupon in a third stage the desolventized earth is subjected to a partially oxidative heat treatment as a result of which the last traces of solvent are removed and organic material that had not been extracted is pyrolyzed to form a carbon deposit, whereupon, in a fourth stage, the carbon coated product is re-activated with for instance hydrochloric acid. In a fifth stage the activated product is washed with water and finally, in a sixth stage the washed product is dried. Oil is recovered from the solvent by solvent evaporation and the solvent thus recovered is recycled. The dried product can be used for bleaching edible oils and is effective in reducing oil color. However, the many stages of this process require a high investment to process relatively small amounts of spent bleaching earth.
The above methods involve the cost of solvent recovery units and anti-explosion provisions. In view of these high costs, it is not surprising that these processes have not been adopted in practice.
In U.S. Pat. No. 2,506,542 a method is disclosed involving the passage of the finely divided spent earth serially through a distilling zone wherein it is heated in the absence of oxygen to a temperature sufficient to distill off and pyrolyze constituents, a burning zone where the remaining carbonaceous constituents are burned off and a cooling zone. The adsorbent is passed through each zone as a horizontally moving bed which is maintained in a fluidized condition by the passage of a suitable gas upwardly therethrough. In each zone the mass is subjected to indirect heat transfer with a suitable heat exchange fluid. However, external heating of fluidized bed in the distilling zone and the burning zone, and the use of an inert gas free of any oxygen in the distilling zone to maintain the fluidized conditions make this process very costly, as a result of which it is hardly operated industrially if at all.
A similar method is disclosed in German Patent specification No. 1,916,692 and is said to involve two separate furnaces, the first one being heated by external heat transfer to pyrolyze the organic matter for about 90 percent, and the second furnace in which the remaining organic matter is burned off. Again, the external heating of the first furnace makes this process commercially impractical.